Recovery of oil



5H H ROOF/ OH 191 June 29, 1965 R. E. GlLcHRlsT 3,191,675

RECOVERY OF OIL Filed June 30. 1961 2 Sheets-Sheet;

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United States Patent O 3,191,675 RECOVERY OF OIL Ralph E. Gilchrist,Bartlesville, Okla., assignor to Phillips Petroleum Company, acorporation of Delaware Filed June 30, 1961, Ser. No. 120,999 13 Claims.(Cl. 166-9) This invention relates to a process for recovering oil froman oil-bearing reservoir by miscible phase displacement. A specificaspect of the invention pertains to a method of establishing atransition zone between a viscosity reducer and the reservoir oil.

The recovery of oil by miscible phase displacement is becoming moreimportant in the oil industry. This process involves establishing atransition zone in the reservoir between the oil and .a viscosityreducer, the transition phase of the drive being forced through thereservoir by the viscosity reducer, usually with the aid of a drivingfluid injected behind the viscosity reducer and transition phase. Theproblem of lingering arises when the viscosity reducer is forced thruthe reservoir structure to displace and produce the reservoir oil. Theterm fingering is used to designate the ow of driving fluid (viscosityreducer) more rapidly thru certain sections of the reservoir toward theproduction well where it is produced prematurely. This is due at leastin part to too much and t-oo sharp a difference in viscosity between thedisplacing uid and the reservoir oil. This signifies improper formationof the transition zone.

Two different methods of establishing transistion zones in miscibleooding or uid drive processes have been practiced. One of these,illustrated in U.S. Patent 2,742,- 089, involves injecting a singlemiscible liquid hydrocarbon slug, such as L.P.G., into the reservoirwith the resultant transition zone forming some distance away from theinjection well. The effect of this transition zone on production isillustrated in FIGURE l, curve A, of the drawing referred tohereinafter. The early breakthrough plus uneven transition zone,indicates the severity of the lingering.

U.S. Patent 2,867,277 illustrates a second method of establishing atransition zone in a miscible phase displacement process. The process ofthis patent involves premixing the hydrocarbon fluids in variableproportions at the well head to establish a broad transition zone in theinjection stream, itself, before it passes into the stratum. The effecton production by this method of establishing the transition zone isrepresented by curve B. It should be noted that the transition zonebreakthrough occurs later than in the process of the aforesaid patent,and a longer transition zone results. However, the uneven transitionzone still reflects the severity of the fingering.

This invention is concerned with .a method of establishing a transitionzone which is more effective in eliminating the problem of fingering andproducing higher yields of oil from the reservoir.

Accordingly, it is an object of the invention to provide an improvedprocess for producing oil from an oil-bearing reservoir, utilizingmiscible fluid displacement or drive. Another object is to provide amethod of establishing a transition zone between the reservoir oil andthe displacing medium. A further object is to provide a process forrecovering oil by miscible fluid displacement which decreases lingering.Other objects of the invention will become Iapparent to one skilled inthe art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises establishing the transitionzone between the reservoir oil and the miscible fluid used inthe processby alternately injecting separate portions of heavy oil, compatible withthe reservoir oil and with the miscible uid, and miscible Huid or Vis-ICC cosity reducer, whereby the initial laminar flow stream of themiscible viscosity reducer and oil mixes the constituents as it is movedthrough the reservoir to form the transition zone of substantiallygradually diminishing viscosity upstream of flow. A preferred embodimentof the invention comprises determining the volume yof the miscibleviscosity reducer to be injected and dividing this volume into separateportions of gradually diminishing volume and injecting these separateportions in order, from the smallest to the largest, alternately withseparate gradually decreasing portions of heavy oil. The total volume ofoil and viscosity reducer are preferably about the same but may varysomewhat. The heavy oil is preferably crude oil from the reservoir beingproduced; however, other heavy oil or crude oil from `another reservoirmay be utilized providing the oil is compatible with the crude of thereservoir being produced and with the viscosity reducer with which it isbeing Iassociated. The term compatible is understood to mean that mixingof the oil with the viscosity reducer and with reservoir oil forms asolution and does not cause a precipitate.

The volume 'of miscible viscosity reducer to be utilized in the methodor process is determined in relation to the character of the reservoirand reservoir oil to be produced. This volume usually is in the range of0.5 to 10% of the effective pore volume of the reservoir in thedisplacement flow path. In reservoirs containing oil of high APIgravity, greater well spacing is permissible and the volume of viscosityreducer will generally be in the lower portion of the range of porevolume given; whereas, in reservoirs containing oil of low API gravity,the volume of viscosity reducer is generally in the upper portion of therange.

Commonly used viscosity reducers comprise low boiling hydrocarbons suchas C2C5 hydrocarbons, L.P.G. (including principally ethane, propane andbutane), kerosene, and mixtures or blends of normally liquidhydrocarbons of substantially lower viscosity than the viscosity of thereservoir oil to be produced.

After calculating the volume of viscosity reducer to be injected intothe reservoir, the number -of separate portions into which the totalvolume is to be divided is determined. This number should be infinitefor the best theoretical results, but is limited by economics andpracticality. Usually at least 4 or 5 portions are separately injectedalternately with the heavy oil .and the injection of a greater number ofportions is advantageous in most reservoirs. The use of 2O or moreportions of gradually increasing volume is advantageous.

A similar second transition zone is established in a similar mannerbetween the miscible viscosity reducer and the driving fluid injectedbehind same when such a driving fluid is used. Natural gas or methanefunctions as an effective driving fluid for L.P.G., and L.P.G. functionsas an effective driving fluid for heavier liquid hydrocarbons such askerosene, Soltrol, and Stoddard solvent. The second transition zone isformed by injecting separate slugs or portions of the driving fluidalternately with separate portions of the miscible viscosity reducer insimilar manner to the injection of the constituents of the lirsttransition zone, using increasingly larger portions of driving uid andgradually decreasing portions of viscosity reducer.

A more complete understanding of the invention may be had by referenceto the accompanying schematic drawing of which FIGURE l is a graphshowing curves obtained by plotting percent of oil in the efiluentagainst the time in seconds for production of uids, utilizing asimulated oil sand described hereinafter, with 3 different techniquesfor establishing the transition zone; FIGURE 2 is an exploded view ofthe apparatus used in the tests on which the graphs are based; andFIGURE 3 is a vertical partial section through a reservoir illustratinglthe process and method of the invention.

1n FIGURE 1, curve A illustrates the results obtained by injecting theentire slug or propane so that there is an instantaneous change from100% oil and 0% propane to 100% propane and 0% oil, as practiced in U.S.Patent 2,742,089.

Curve B represents the results obtained when operating in accordancewith U.S. Patent 2,867,277. With this method, there is a gradual changein composition lfrom two pumps (in 5% increments), with each changebeing held the same number of time units (in this case 480 sec- Curve Crepresents operation in accordance with the invention. In this processthere is a gradual change in composition from two pumps with one pumpoff while the other pump is running. The change in time units is thesame as in the operation associated with curve B, i.e., 480 seconds. Thetime that each pump runs varies in 5% increments. The total rate of flowof both pumps is the same as used in connection with curves A and B. Theschedule of operation of the pumps is given below:

Time, sec. Propane, sec. Oil, sec.

96 (0. 20)(480) 120 (0. 25)(480) 144 (0. 30)(480) 168 (0. 35 X480) 192(0. 40 X480) 216 (0. 45 480) 240 (0. 50)(480) 264 (0. 55X480) 288 (0.00)(480) 312 (0. 65 X480) 336 (0. 70)(480) 360 (0. 75X480) 384 (0.80X480) 408 (0. 85)(480) 432 (0. 90X480) 456 (0. 95 X480) 480 (1.00X480) FIGURE 2 shows the apparatus utilized in obtaining the data forcurves A, B, and C of FIGURE 1. The device comprises a metal frame 10,1l in thickness and having a cutout section 12 for a sand compartmentwhich is 3" wide and 9" long and is provided with an inlet 14 at one endand an outlet 16 at the opposite end. Both the inlet and outlet surfacesof frame adjacent space 12 are provided with grooves 18 leading alongthe inner face of the end of the frame for distribution of fluid overthe entire end and pickup of fluid over the entire opposite end adjacentoutlet 16. A screen 20 is placed over the inlet end, and a similarscreen (not shown) is placed over the outlet end of the frame. Space 12is filled with sand after is provided with a space 26, 1A deep, forexerting pressure on the rubber gasket 22 by injecting nitrogen or otherpressurizing gas into space 26 through conduit 28. A rubber gasket 30 isplaced around the top `surface of plate 10 and coverplate 32 having awindow 34 therein is positioned on the gasket 30. A `series of 24 bolts36 (only one being shown), with matching nuts and washers 38 and 40,respectively, bolt the assembly together to simulate a section of sand.Valved line 42 connects inlet 14 with the propane pump and valve line 44connects line 14 with the oil pump. Before simulated starting offormation of the transition zone, the sand in space 12 is saturated withcrude oil. The pumps are then operated in accordance with the schedulesfor curves A, B, and C for the different tests or runs, and the effluentin line 16 is periodically analyzed for composition to obtain oil andpropane concentration. Variations in the oil concentration in theeffluent clearly show the result of fingering of the miscible transitionzone, particularly with respect to curves A and B. Curve C shows muchless fluctuation in the oil content of the produced effluent and,therefore, less fingering of the displacing fluid and a more efficienttransition zone.

Referring to FIGURE 3, an oil stratum or reservoir 46 is penetrated byan injection well 48 and a production well 50. Wells 48 and 50 areprovided with casings S2 and 54, respectively, and with tubing strings56 and 58, respectively. The annulus in well 48 within reservoir 46 ispacked with sand or gravel 60 and a packer 62 is set around tubing 56 inthe lower end of the casing. The packing of sand or gravel 60 in thewell around the tubing string assures mixing of the injected fluidswithin the reservoir sand face surrounding well 48. The section oftubing 60 below packer 62 may be perforated to provide betterdistribution of fluids in the gravel. Also, casing 52 may extend to thebottom of the hole, being perforated below packer 62 before packing thehole.

Lines 64, 66, and 68 connect with tubing 56 and with hsuitable pumps toprovide for injection of L.P.G., oil, and natural gas, respectively.

An illustration of the laminar nature of the transition zone is shownbetween the depleted reservoir 70 and the oil-filled reservoirintermediate the transition zone and the production Well 50. Thisillustration assumes that the injected constituents of the transitionzone remain in their respective positions without mixing in order tobetter demonstrate the size and the order of the portions injected.Actually, the portions of oil and L.P.G. first injected thru well 48begin to mix as they are forced deeper into the formation, as do thelater injected portions, so that there is a substantially gradualdecrease in viscosity upstream of flow in the reservoir or toward theinjection well.

In the case illustrated in FIGURE 3, the total volume of the L.P.G. slugto be injected is calculated and this amount is -divided into 3 units,0.2 unit being injected as the first portion, 0.4 unit as the Secondportion, 0.6 unit as the third portion, 0.8 unit as the fourth portion,and 1.0 unit as the fifth portion. A similar volume of crude oil isdivided into 3 units and injected in portions alternately with theL.P.G. and in reverse order of the size of the portions, beginning withthe largest (1.0) and ending with the smallest (0.2). Following theinjection of the crude oil and L.P.G. slugs forming the first transitionzone, the methane (or natural gas) and L.P.G. slugs forming the secondtransition zone are injected in similar order and manner.

Variations in the technique illustrated are within the scope of theinvention, it being feasible to inject slugs of propane of equal volumeinterspersed or alternated with oil slugs -of substantially decreasingvolume. It is also feasible to inject slugs of oil of equal volume andto progressively increase the volume of the injected slugs of L.P.G. Thesame variations apply to the formation of the second transition zone. Itshould be realized that if the portions of injected fluids are extremelysmall, the technique really amounts to injecting the Viscosity reducerand reservoir oil simultaneously at diiferent rates, one lluid atgradually increasing rates and the other at gradually decreasing rates.With this technique, the fluids are simultaneously injected into thetubing string thru a connection or mixing chamber at the well head.

In the disclosure thus far it has been assumed that the rst sluginjected consists of heavy oil or crude oil from the reservoir beingproduced. This practice of injecting the oil, rst, is advisable when thepores of the reservoir are not substantially lled with oil at thebeginning of the production process. In the event the process is beingapplied to a virgin reservoir or to one in which the reservoir pores aresubstantially lled with crude, particularl1y around the injection well,it is feasible to inject the small slug of L.P.G. irst, followed by thealternate injection of oil and L.P.G. as described hereinbefore.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim:

1. In a miscible phase displacement process for producing oil from anoil-bearing reservoir penetrated by an injectionWelland,a.prodntinnnellowherein a quantity of viscosity re ucer misciblewith the reservoir oil is injected thru said injection well into saidreservoir to establish a transition zone between the oil and saidviscosity reducer and drivingqglidisgipjectedmthruaidu@ jection wellbehind s,a l i d yj&c v4 reducer to drive same,

along with oil, thru said reservoir toward said production well,thgirnpkrgvemenlornp'rising establishing said transition zone byinjecting said gjnar'itityTfr"visonsityledugpF Y`saidnziscosity. reducensaid portion vof `yrsc lgrswlty"reducer increasing gradually in volumefrom a portion less than 0.25 of the volume of the first portion of oiland the portions of oil gradually decreasing in volume until the lastportion of Oil is less than 0.25 of the volume of the adjacent portionof viscosity reducer.

2. The process of claim 1 wherein said viscosity reducer comprisesliquid hydrocarbon of substantially lower Viscosity than said reservoiroil and the injected oil is said reservoir oil.

3. The process of claim 2 wherein said hydrocarbon is liqueed gas.

4. The process of claim 1 wherein the volume of said viscosity reduceris in the range of 0.5 to of the effective pore Volume of the reservoirin the displacement ow path.

S. The process of claim 1 including the steps of establishing a secondtransition zone between said viscosity reducer and said driving fluidcomprising injecting said driving fluid in separate portions alternatelywith additional separate portions of said viscosity reducer.

6. The process of claim 1 wherein said viscosity reducer consistsessentially of liquid hydrocarbon of substantially lower viscosity thansaid reservoir oil, the injected oil consists essentially of reservoiroil and said driving fluid consists essentially of normally gaseoushydrocarbons.

7. The process of claim 6 wherein a second transition zone between saidliquid hydrocarbon and said normally gaseous hydrocarbon is establishedby injecting said gaseous hydrocarbon in separate portions alternatelywith separate additional portions of said liquid hydrocarbon, theportions of said normally gaseous hydrocarbon increasing gradually inVolume from less than 0.25 of the volume of the adjacent portion ofliquid hydrocarbon and the additional portions of liquid hydrocarbondecreasing gradually in volume until the last portion thereof 6 is lessthan 0.25 of the volume of the last portion of said normally gaseoushydrocarbon.

8. The process of claim 7 wherein the volume of said liquid hydrocarbonin each transition zone is in the range of 0.5 to 10% of the elfectivepore volume of the reservoir within the displacement llow path.

9. In the production of oil from an oil-bearing reservoir by misciblephase displacement, the method of establishing a transition zone betweenthe reservoir oil and a viscosity reducer comprising injecting into saidreservoir thru an injection well therein separate portions of saidviscosity reducer alternately with separate portions of crude oilcompatible with the reservoir oil and with said viscosity reducer, saidportions of viscosity reducer successively increasing in volume and saidportions of oil successively decreasing in volume, whereby the initiallaminar flow stream of viscosity reducer and oil mixes theseconstituents as it is moved thru said reservoir to form a transitionzone of gradually diminishing viscosity upstream of ow.

10. The method of claim 9 wherein the portions of viscosity reducergradually increase in volume from a portion less than 0.25 of the volumeof the irst portion of oil and the portions of oil gradually decreaseuntil the last portion of oil is less than 0.25 volume of the adjacentportion of viscosity reducer, the total volume of viscosity reducerbeing in the range of 0.5 to 10% of the effective pore Volume of thereservoir in the displacement ow path.

11. In a miscible phase displacement process for producing oil from anoil-bearing reservoir penetrated by an injection well and a productionwell wherein a quantity of viscosity reducer miscible with the reservoiroil is injected thru said injection well into said reservoir toestablish a transition zone between the oil and said viscosity reducerand driving uid is injected thru said injection well behind saidviscosity reducer to drive same, along with reservoir oil, thru saidreservoir toward said production well, the improvement comprisingestablishing said transition zone by injecting said quantity ofviscosity reducer in separate portions alternately with separateportions of heavy oil compatible with the reservoir oil and with saidviscosity reducer, said separate portions of viscosity reducersuccessively increasing in volume and said separate portions of oilsuccessively decreasing in volume.

12. In a miscible phase displacement process for producing oil from anoil-bearing reservoir penetrated by an injection well and a productionwell wherein a quantity of viscosity reducer miscible with the reservoiroil is injected thru said injection well into said reservoir toestablish a transition zone between the oil and said viscosity reducerand driving liuid is injected thru said injec- -tion well behind saidviscosity reducer to drive same, along with reservoir oil, thru saidreservoir toward said production well, the improvement comprisingestablishing said transition zone by dividing said quantity of viscosityreducer into separate portions of gradually diminishing volume andinjecting these separate portions in order, from the smallest to thelargest, alternately with separate gradually decreasing portions ofheavy oil.

13. The process of claim 12 wherein the total volumes of injectedviscosity reducer and heavy oil are approximately equal.

References Cited bythe Examiner UNITED STATES PATENTS 2,867,277 1/59Weinaug et al 166--9 2,875,831 3/59 Martin et al. 166-9 2,927,637 3/60Draper 166-9 3,080,917 3/63 Walker 166-9 CHARLES E. OCONNELL, PrimaryExaminer.

1. IN A MISCIBLE PHASE DISPLACEMENT PROCESS FOR PRODUCING OIL FROM ANOIL-BEARING RESERVOIR PENETRATED BY AN INJECTION WELL AND A PRODUCTIONWELL W HEREIN A QUANTITY OF VISCOSITY REDUCER MISCIBLE WITH THERESERVOIR OIL IS INJECTED THRU SAID INJECTION WELL INTO SAID RESERVOIRTO ESTABLISH A TRANSITION ZONE BETWEEN THE OIL AND SAID VISCOSITYREDUCER AND DRIVING FLUID IS INJECTED THRU SAID INJECTION WELL BEHINDSAID VISCOSITY REDUCER TO DRIVE SAME, ALONG WITH OIL, THRU SAIDRESERVOIR TOWARD SAID PRODUCTION WELL, THE IMPROVEMENT COMPRISINGESTABLISHING SAID TRANSITION ZONE BY INJECTING SAID QUANTITY OFVISCOSITY REDUCER IN SEPARATE PORTIONS ALTERNATELY WITH SEPARATEPORTIONS OF HEAVY OIL COMPATIBLE WITH THE RESERVOIR OIL AND WITH SAIDVISCOSITY REDUCER, SAID PORTION OFVISCOSITY REDUCER INCREASING GRADUALLYIN VOLUME FROM A PORTION LESS THAN 0.25 OF THE VOLUME OF THE FIRSTPORTION OF OIL AND THE PORTIONS OF OIL GRADUALLY DECREASING IN VOLUMEUNTIL THE LAST PORTION OF OIL IS LESS THAN 0.25 OF THE VOLUME OF THEADJACENT PORTION OF VISCOSITY REDUCER.